Remove Markdown-it from content editor build

app/assets/javascripts/content_editor/services/markdown_serializer.js

 import {
   MarkdownSerializer as ProseMirrorMarkdownSerializer,
   defaultMarkdownSerializer,
-} from 'prosemirror-markdown';
+} from 'prosemirror-markdown/src/to_markdown';
 import { DOMParser as ProseMirrorDOMParser } from 'prosemirror-model';
 
 const wrapHtmlPayload = (payload) => `<div>${payload}</div>`;

jest.config.base.js

@@ -101,7 +101,7 @@ module.exports = (path, options = {}) => {
       '^.+\\.(md|zip|png)$': 'jest-raw-loader',
     },
     transformIgnorePatterns: [
-      'node_modules/(?!(@gitlab/ui|@gitlab/favicon-overlay|bootstrap-vue|three|monaco-editor|monaco-yaml|fast-mersenne-twister)/)',
+      'node_modules/(?!(@gitlab/ui|@gitlab/favicon-overlay|bootstrap-vue|three|monaco-editor|monaco-yaml|fast-mersenne-twister|prosemirror-markdown)/)',
     ],
     timers: 'fake',
     testEnvironment: '<rootDir>/spec/frontend/environment.js',